1. Field of the Invention
The present invention relates generally to an oscillating force generator suitably used in a vibration damper which is installed on a subject body or a member of a vibration system so as to actively exhibit an excellent vibration damping effect with respect to vibrations to be damped, that is, the vibrations generated in the subject body or transmitted through the member of the vibration system. More particularly, the present invention is concerned with an oscillating force generator used in an active-type vibration damping device and having a novel structure, and an active-type vibration damper equipped with the oscillating force generator of the invention.
2. Description of the Related Art
As means for damping or reducing vibration of a subject body, there has been employed a vibration damping device which is capable of reducing the vibration of the subject body owing to a vibration damping effect of a shock absorber, an elastic body or the like, and a vibration isolating device which is capable of reducing the vibration of the subject body owing to spring characteristics of a coil spring, an elastic body, or the like. These vibration damping and isolating devices are adapted to exhibit passively their vibration damping or isolating effect with respect to the vibration to be damped. Such a passive-type vibration damping or isolating device suffers from difficulty in exhibiting a sufficient vibration damping effect, when the vibration to be damped changes its vibration characteristics. To overcome this drawback, there has been developed an active-type vibration damper. The active-type vibration damper has an actuator adapted to generate an oscillating force corresponding to the vibration to be damped and apply the oscillating force to the subject body, thereby actively offsetting or absorbing the vibration to be damped.
In such an active-type vibration damper as described above, it is required to employ an oscillating force generator capable of generating an oscillating force whose frequency is suitably controllable. Examples of such an oscillating force generator are disclosed in U.S. Pat. No. 5,427,362 and JP-A-8-312718, wherein an electromagnetic drive device of a voice-coil type is employed as the oscillating force generator. The voice-coil type electromagnetic drive device includes a permanent magnet having opposite magnetic pole faces which are opposite to each other, and a moving coil disposed between the opposite pole faces of the permanent magnet. The moving coil is energized by a controlled electric current, whereby the coil is subject to a Lorents force or an electromagnetic force, so that the coil is moved to provide the desirably controlled oscillating force.
However, the conventional oscillating force generator of voicecoil type is likely to generate a relatively small oscillating force. For generating the desired oscillating force which is large enough to assure a high damping effect, the voice-coil type oscillating force generator tends to be large sized, and the electric power consumed by the generator is inevitably increased. The voice-coil type oscillating force generator also suffers from a problem of heat generated therein. While the moving coil and the permanent magnet is displaced relative to each other in the axial direction thereof upon energizing the moving coil, the moving coil and the pole faces of the permanent magnet are likely to be sliding contact with each other, causing undesirable noise, loss of energy, and damage to the contact parts of the moving coil and the permanent magnet.
Another type of oscillating force generator is disclosed in JP-A-10-246283 that is suitably used of the active-type vibration damping device, wherein an electromagnet is utilized. Such an electromagnet-type oscillating force generator includes a yoke member made of a magnetic material and having an annular groove open in one of its axially opposite end faces, and a coil accommodated in the annular groove of the yoke member. Upon energizing the coil by application of an electric current thereto, there is generated a magnetic path or circuit around the coil, so that the inner and outer wall portions of the annular groove of the yoke member are magnetized so as to have respective opposite magnetic poles or pole faces, on their open end portion. Further, the electromagnet-type oscillating force generator includes an oscillating member made of a magnetic material which is disposed axially opposite to the open end faces of the inner and outer wall portions of the yoke member with a given axial distance therebetween. In this condition, the coil is energized so that the oscillating force generator generates an electromagnetic force between the oscillating member and the yoke member in the axial direction thereof. This electromagnetic force acts on the oscillating member as an axial oscillating force, causing an axially reciprocal movement of the oscillating member.
This oscillating force generator of electromagnet-type permits a high-precise control of the oscillating force in terms of its frequency, phase or the like, by controlling the electric current applied to the coil. Further, the electromagnet-type oscillating force generator is capable of generating a sufficiently large oscillating force in comparison with the voice-coil type oscillating force generator.
In the conventional oscillating force generator of electromagnet type, however, the pole faces of the yoke member and the oscillating member are arranged to be directly opposed to each other in a direction in which these two members are displaced relative to each other, that is, in the axial direction thereof, with the predetermined axial distance therebetween. In this arrangement, the magnitude of the oscillating force generated by the oscillating force generator is significantly influenced by an amount of spacing between the yoke member and the oscillating member. Therefore, a slight difference in an initial position of the oscillating member relative to the yoke member may cause fail in generating desired oscillating force and a resultant fail in exhibiting a sufficient vibration damping effect. Thus, the conventional oscillating force generator is incapable of generating desired damping characteristics with stability.